Impact of stellar companions on precise radial velocities

Context: With the announced arrival of instruments such as ESPRESSO one can expect that several systematic noise sources on the measurement of precise radial velocity will become the limiting factor instead of photon noise. A stellar companion within the fiber is such a possible noise source. Aims: With this work we aim at characterizing the impact of a stellar companion within the fiber to radial velocity measurements made by fiber-fed spectrographs. We consider the contaminant star either to be part of a binary system whose primary star is the target star, or as a background/foreground star. Methods: To carry out our study, we used HARPS spectra, co-added the target with contaminant spectra, and then compared the resulting radial velocity with that obtained from the original target spectrum. We repeated this procedure and used different tunable knobs to reproduce the previously mentioned scenarios. Results: We find that the impact on the radial velocity calculation is a function of the difference between individual radial velocities, of the difference between target and contaminant magnitude, and also of their spectral types. For the worst-case scenario in which both target and contaminant star are well centered on the fiber, the maximum contamination for a G or K star may be higher than 10 cm/s, on average, if the difference between target and contaminant magnitude is $\Delta m$ < 10, and higher than 1 m/s if $\Delta m$ < 8. If the target star is of spectral type M, $\Delta m$ < 8 produces the same contamination of 10 cm/s, and a contamination may be higher than 1 m/s